The invention relates to a method of forming a porous structure on the surface of a solid substrate. The porous structure is created by the use of a microstructure appliqué consisting of a binder and a particulate material formed in the predetermined shape and depth and applied to a substrate of similar or dissimilar material and geometrical configuration. The process is especially, although not exclusively, useful for making prosthetic implant devices with porous metal-coated surfaces.
Various methods have been disclosed in the literature for providing a porous metal coating on the surfaces of prosthetic implants for securing such devices by means of both soft and hard (bone) tissue ingrowth.
In one such method disclosed by Hahn in U.S. Pat. No. 3,605,123, a porous surface structure is built up by applying multiple metal layers using a flame spray process. This technique was found to be undesirable due to the weak interfacial adhesion to bone tissue. Later techniques by Pillar (in U.S. Pat. No. 3,855,638) and others (see patent references) formed the porous metal coating on implant devices by applying a slurry of metallic powder suspended in aqueous solution with organic binders. The applied slurry layer is converted to the porous coating through diffusion bonding of the metallic particles to one another and to the substrate surface, thereby creating substantially uniform pores and pore size distribution. In U.S. Pat. No. 4,017,911, Kafesjian teaches a method where an adhesive is applied to select areas of a valve body casting followed by pouring metallic powder of specific size onto the treated areas. Additional layers of powder are applied by repeating the process until reaching the desired thickness. After achieving the desired depth, the coated casting is sintered in an appropriate atmosphere to permanently attach the particles to the casting and to each other. The shape and size of the metallic powder and the appropriate choice of sintering conditions control the porosity of the sintered layer.
In U.S. Pat. No. 4,536,894 by Galante et al, the application of prefabricated porous pads secured to flat surfaces or pressed into hollow depressions on the stem of a hip prosthesis is mentioned but no method is given for fabricating the porous pads.
Porous coatings are again added to bone prosthesis in U.S. Pat. No. 4,550,448 where a layer of spherical metal powder is deposited on adhesive-coated areas, via treatment in a fluid bed, and presintered to burn off the binder and establish bonding between different particles and between particles and the substrate surface. A second layer of particles is added on top of the first layer in similar fashion and presintered a second time for establishing secure bonding between layers. A third sinter step is performed on the coated substrate to promote formation of secure junctions between particles, between layers and attachments to the substrate. Several drawbacks to using such a method involving multiple high-temperature heat treatments would be the addition of significant time and costs to the manufacturing cycle as well as the potential to degrade both mechanical properties and material properties and distort prosthesis dimensions.
Yet in another method called out in U.S. Pat. No. 4,612,160, a porous metal coating is created by positioning a rigid mold in close proximity to a substrate such that the space between mold and substrate defines the boundaries of the porous layer. The defined space is filled with metal powder, without a binder, and the mold/substrate assembly then presintered to lightly bind particles together and to the substrate. After removal from the mold, the coated substrate is further sintered to obtain the proper desired bond strength and pore volume. This technique would be difficult to implement for coating areas with complex geometrical shapes and patterns but offers a way to eliminate sources of contamination introduced through the use of adhesive binders. In a related method, Bugle, in U.S. Pat. No. 4,854,496, describes a method where a porous pad of pure titanium is first made by presintering powder in a cavity of the desired shape. The shaped pad then sintered to achieve required bond strength, the pad flattened on one side to create a bonding surface and finally sintered to the substrate under application of pressure within a non-reactive atmosphere.
Chowdhary describes a method in U.S. Pat. No. 5,104,410 wherein a titanium powder is combined with a urea to form a mixture that is compressed into pockets using a hydraulic press. After compaction, the urea component is leached out of the pressed-on material by soaking the device in water and subsequently dried and sintered in vacuum. The porosity of the added layer is created by the removal of urea within the volume of the compacted pockets.
In a series of patents by Devanathan et al, the inventor describes the application and fixation of porous surfaces to an implant using laser welding. In U.S. Pat. No. 5,773,789, porous metal pads are fabricated using a sintering or diffusion bonding process. Pre-shaped pads with adhesive backing are placed on the implant and coupled to the surface at a plurality of locations by application of a laser beam to form weld beads attaching the pads to the implant body.
Finally, metallic porous bead preforms are described in U.S. Pat. Nos. 6,193,761 and 6,209,621. Here, the bead preforms are first prepared by mixing metallic beads with binder, such as methylcellulose, and applying the resultant slurry to a mold of the desired shape. The bead/binder slurry is fired to burn off the binder and bond beads together. The shaped preform is then attached to the prosthesis during its formation by the casting of molten metal into a refractory shell containing said preform positioned where a porous metal coating is desired on the implant device.